In recent years, press-working has become more automated and higher in efficiency. Multi-process press machines which using single systems to simultaneously perform a plurality of shaping operations are becoming more popular. This type of press has a plurality of bottom dies arranged on a bed. A crankshaft is rotated to raise and lower slides from which top dies are suspended in the shaping order. A workpiece is conveyed by a conveyor system. In combination, automated press-forming work is performed. That is, a workpiece is clamped and pressed between the dies of a first process. When one shaping operation is completed, the conveyor system is operated to move the workpiece horizontally to the next adjoining dies where the next shaping operation is performed. By repeating this procedure, a single press machine can be used to perform a shaping operation with a large working rate through a plurality of processes in an integrated manner, so processing with a high work efficiency and good productivity is realized.
However, in a conventional multi-process press machine, linked with rotation of a single crankshaft, connecting rods engaged with eccentric parts are simultaneously raised and lowered and slides hanging from their bottoms are raised and lowered to press several aligned workpieces all at once. For this reason, at this time, the load applied to the press machine becomes the total of all of the working loads in the plurality of dies. If assuming a single press machine provided with three types of dies and performing three processes of press-forming simultaneously, if the load occurring at the first process is K1, the load occurring at the second process is K2, and the load occurring at the third step is K3, since these press-forming operations proceed simultaneously, the total of the loads occurring at the press machine as a whole becomes the total of the individual loads or K1+K2+K3=KT. Naturally, if the number of processes of working increases and further the larger the shaping ratio per process, the larger the total value becomes. To withstand the resultant massive load, the press machine must be made extremely strong in rigidity. In the end, the press machine becomes massive, the required installation area becomes greater, and the ancillary facilities also become large in size. Further, in recent press machines, the shaped products are becoming increasingly diverse in type. Even the products shaped by a single machine come in thousands and tens of thousands of types. For this reason, in a massive press machine, frequently the generally excessive capacity detracts from the universal applicability. In particular, if producing a small number of pressed products which can be formed by a light load by a massive facility, the loss becomes too great and the prime units of energy rise resulting in lower productivity. On top of the prime units of production, there is a high risk of a large detrimental effect being felt. In such a case, it becomes necessary to provide separate light load press machines in parallel.
Press machines solving such problems, that is, press machines relatively small in size and sufficiently economic in structure despite performing multiple processes, are disclosed in PLTs 1 and 2. For example, the art disclosed in the PLT 2 is as follows: That is, this provides a multi-process press machine providing a single press machine with three dies and performing three processes. In this press machine, the eccentric parts of the crankshaft do not have a single eccentric axis, but are provided with three eccentric axes shifted in phase. For this reason, at the instant at which the first process is being performed, the second and third processes are not being executed. The load applied to the press machine as a whole at this time becomes just the load K1 of the first process. After the first process ends and the crankshaft rotates by exactly a predetermined angle, the second process is executed. The load applied to the press machine as a whole at this instant is just the load K2 of the second process. When the second process ends and the crankshaft rotates by exactly a further predetermined angle, the third process starts and the load K3 is generated. In this way, the press-working operations are performed consecutively offset in time, so for the maximum load applied to the press machine as a whole, it is sufficient to assume the largest of the individual loads, that is, K3.
However, in the above-mentioned multi-process press machine, while press-working a workpiece, sometimes the workpiece will move or warp or the workpiece will partially crack resulting in a large drop in the quality of the final processed product. Such trouble is believed to occur, first, because the workpiece is a strip-shaped material and the workpiece has to be conveyed between a plurality of processes, so such a phenomenon occurs at the time of plastic-working.